If by "log in" you mean using the linuxmce user, I have seen this before. The MD is treated as an appliance, best I can tell, and for the user linuxmce to connect to it, it uses credentials almost like a SAMBA CIFS share. It does not have a passwd file on hand for a specific user, just the sambahelper user, and root. Again this is mostly speculation on my part, but I have tried this since 7.04 and I got the same result as you. I usually sudo to root on the core, and then ssh out from there to my MD's. As to it being a bug, of this i am not sure, as it has been the same for me since I started.
Perhaps one of the more learned sages here at LMCE can better address that.
But I am glad it worked for you.